An ion exchange membrane is used as a membrane for electrodialysis in salt production and for removal of salts from underground brine, food and the like. In the process of electrodialysis, ions move when a direct current is applied to electrodes in an electrodialysis vessel in which anion exchange membranes and cation exchange membranes are alternately arranged and fastened between the electrodes, i.e., cathode and anode. Accordingly, desalting compartments where the ion concentration decreases and concentration compartments where the ion concentration increases are alternately placed. Typically with continued electrodialysis, the concentration of a hardly-soluble salt is increased in the concentration compartment so that scaling occurs due to precipitation of the salt. In order to prevent scaling, a chemical substance such as an acid is added in the concentration compartment, however, there is a problem that a large amount of the chemical substance has to be used.
Upon this problem, electrodialysis reversal (Electro Dialysis Reversal: EDR) was developed in order to prevent scaling by reversing the polarity of electrodes before the salt concentration in the concentration compartment reaches a critical concentration for occurrence of scaling. This method enables to remove deposits on an ion exchange membrane generated in the normal phase operation from the ion exchange membrane by means of a repulsive force of charges during reverse phase operation.
Patent Document 1 (WO 2010-119858) discloses an ion exchange membrane in which an ion exchanger composed of a block or graft copolymer comprising a cationic or anionic copolymer component and a vinyl alcohol copolymer component is formed on a supporting layer by printing.
Patent Document 1 discloses the following features for the ion exchange membrane.
(1) Fouling resistance: the problem on organic contamination resistance (fouling resistance) may be solved by including as a constitutional unit a vinyl alcohol polymer that is a hydrophilic polymer.
(2) Mechanical strength: mechanical strength is imparted to the ion exchange membrane by locating the ion exchange membrane on a supporting layer.
(3) Electrical properties (resistance): electrical properties may be imparted to the ion exchange membrane by providing a copolymer comprising a vinyl alcohol polymer and an ionic group-containing polymer.
Patent Document 2 (JP Laid-open Patent Publication No. 2012-40508) discloses an ion exchange membrane including a nonwoven fabric sheet, and an ion exchange resin-coated layer provided on one surface of the nonwoven fabric sheet. The nonwoven fabric sheet has a fiber layer structure comprising a fine fiber layer having a fiber diameter of 5 μm or less as an intermediate layer and continuous fiber layers each having a fiber diameter of 8 to 30 μm on both surface of the fine fiber layer, wherein the fiber layer structure is formed by fiber to fiber fusion.
In Patent Document 2, the fine fiber layer is provided as an intermediate layer between two continuous fiber layers. Thus, even when an inexpensive nonwoven fabric sheet is used, the strength, dimension stability and shape stability can be improved, and the occurrence of undulation caused by contact of the nonwoven fabric sheet with an electrolytic solution is effectively suppressed, so that an ion exchange membrane having low membrane resistance can be obtained.